JONNY PHILLIPS: Dams don't always hold water back, sometimes they have to let it through. For example, if there's a threat to the dam's integrity, or they need to increase supply downriver, then they'll open the valves.

NARRATOR: This valve alone can release over 8000 litres of water a minute.

RICHARD AMBROSE: Now that is a lot of water! Turn it off! Turn it off!

RICHARD AMBROSE: Now there are a few dams in the country where a release of water can be requested by anyone.

JONNY PHILLIPS: You might be a farmer downriver who just needs a bit more water released to irrigate their land.

RICHARD AMBROSE: Or an environmental group concerned that lower water levels downriver are compounding the effects of pollution.

JONNY PHILLIPS: But you do have to pay. For water to be released for a whole day it can cost 1500 pounds.

NARRATOR: But despite the cost, an increasing number of people are requesting the release of water so they can enjoy thrill seeking recreational pursuits!

JONNY PHILLIPS: And one of the most popular of those activities on this river is whitewater rafting. But just how much difference can a release of water from a dam upriver make?

RICHARD AMBROSE: Well we're not moving at all at the moment and the river is just flowing gently, but we've been told they're gonna release a load of water from the dam up above and this river is gonna turn into a monster.

NARRATOR: With the dam open, the amount of water flowing down this river increases to a massive 900 million litres, that's 50 times the normal amount, which means the boys will be travelling down it 10 times faster than normal!

JONNY PHILLIPS: This is the biggest drop of the river.

RICHARD AMBROSE: That's a big one! Oh my word!

JONNY PHILLIPS: Oh my word!

RICHARD AMBROSE: Here's the biggun!

RICHARD AMBROSE: Without that dam up there we couldn't do this today. And more and more places across the UK are taking advantage of dams to have fun! Let's do it again, come on.